A term used to describe the day approximately three days from yesterday, or, the day after tomorrow. with the word "tomorrow" being taken literally as "2-morrow" and thremorrow as "3-morrow". other spellings include: 3morrow , threemorrow.
This is because the 'to' from tomorrow can be read as 'two' as in the number so add 1 and now it's 'three', shorten it and it's 'tre' and stick that in front of morrow and you get the word tremorrow.
you can also up the amount so the day after tremorrow would be fomorrow ect...